The present invention relates to a magnetron used for radio-frequency heating apparatuses, such as microwave ovens.
FIG. 8 is a longitudinal cross-sectional view of a conventional magnetron incorporated into a microwave oven. FIG. 9 is an enlarged longitudinal cross-sectional view illustrating the main parts of the magnetron shown in FIG. 8. In FIGS. 8 and 9, a magnetron 1 comprises a cathode 3 vertically provided along a central axis, an anode cylindrical body 5 coaxially surrounding the cathode 3, an input pole piece 7 provided at the end of a lower opening of the anode cylindrical body 5, a cathode terminal guiding stem 31 projecting from a first metal tube 9 covering the input pole piece 7, an output pole piece 13 provided at the end of an upper opening of the anode cylindrical body 5, a second metal tube 15 covering the output pole piece 13, and a microwave radiating antenna 19 projecting from the second metal tube 15 through an insulating tube 17 made of ceramic.
A plurality of anode vanes 20 (even-numbered anode vanes) radially arranged to face the central axis of the anode cylindrical body 5 are joined to an inner wall surface of the anode cylindrical body 5. Further, a ring engaging concave portion 20a for joining an equalizing ring and a ring inserting concave portion 20b for inserting the equalizing ring without contact are provided at the upper and lower edges of each anode vane 20 in the radius direction of the anode cylindrical body 5, and the concave portions are reverse to each other in arrangement at the upper and lower edges.
Further, one of a small-diameter equalizing ring 22 and a large-diameter equalizing ring 24 both coaxially arranged with the anode cylindrical body 5 is joined to the ring engaging concave portion 20a, so that the anode vanes 20 arranged in the circumferential direction are electrically connected every other vane.
In a first ring-shaped permanent magnet 21 made of ferrite that surrounds the first metal tube 9 and overlaps the surface of an outer edge of the input pole piece 7, one magnetic pole thereof is magnetically connected to the input pole piece 7. In addition, in a second ring-shaped permanent magnet 23 made of ferrite that surrounds the second metal tube 15 and overlaps the surface of an outer edge of the output pole piece 13, one magnetic pole thereof is magnetically coupled to the output pole piece 13.
Furthermore, a frame-shaped yoke 25 for magnetically coupling the other magnetic pole of the first ring-shaped permanent magnet 21 to the other magnetic pole of the second ring-shaped permanent magnet 23 has a through hole 25a for passing through the cathode terminal guiding stem 31 at the lower end thereof.
A plurality of radiating fins 27 are mounted to the outer circumferential surface of the anode cylindrical body 5 in a multi-stage manner, and a metal filter case 29 for preventing the leakage of electromagnetic waves toward the outside of an apparatus is mounted to the outer surface of a lower end of the frame-shaped yoke 25. In addition, the cathode terminal guiding stem 31 having a diameter smaller than that of the through hole 25a of the frame-shaped yoke 25 is tightly soldered to the first metal tube 9. A cathode terminal 11a passes through the cathode terminal guiding stem 31, and the cathode terminal 11a is electrically connected to a lead line 11 electrically connected to the cathode 3.
A through type capacitor 33 is mounted to a side surface portion of the filter case 29, and an end of a choke coil 35 is connected to the cathode terminal 11a of the cathode terminal guiding stem 31 provided in the filter case 29. The choke coil 35 constitutes an LC filter circuit for preventing the leakage of electromagnetic waves, and the other end thereof is connected to a through electrode of the capacitor 33.
In the magnetron 1 having the above-mentioned structure, in order to prevent the noise leakage of a harmonic wave to the microwave radiating antenna 19, a choke ring 37 having a length of about a quarter wavelength in the axial direction is tightly brazed to the second metal tube 15.
Therefore, in the magnetron, there are restrictions for preventing the unnecessary radiation (noise leakage) of a relatively low frequency component in a range of 30 to 1000 MHz, a reference wave component (a band width and a side band level), and a harmonic wave component having a frequency larger than 4 GHz, and particularly, a strict restriction is inflicted on a fifth harmonic wave, which is a harmonic wave component.
Thus, it is difficult to completely clear the restrictions of the unnecessary radiation using only the choke ring 37.
In general, when the spectrum of a reference wave has a good waveform with little side band, the spectrum of an n-order wave (harmonic wave) is also good, so that it is possible to reduce unnecessary radiation. Further, a radius Rp (a distance from a base including a fillet of a deep-drawing tapered portion to the central axis of the magnetron, that is, a distance from an intersection of a virtual extension line of the flat portion and a virtual extension line of the deep-drawing tapered portion to the central axis of the magnetron) of a small-diameter flat portion of a pole piece formed in a funnel shape by deep drawing greatly affects the generation of the side band on the spectrum of the reference wave.
The flat portion of each pole piece 7 or 13 is a flat area close to the end surface of each anode vane 20 for concentrating a magnetic flux on an operation space in the anode cylindrical body 5, and the variation of the reference wave spectrum is shown in FIGS. 10A to 10E when the radius Rp of the flat portion is gradually increased.
Further, when a radius of the outer circumference of the small-diameter equalizing ring 22 is Rs1, a radius of the inner circumference of the large-diameter equalizing ring 24 is Rs2, and a minimum length Lg between upper and lower pole pieces in the axial direction is two-point-eight times as large as a radius Ra of a circle inscribed in a leading edge of the anode vane 20, the radius Rp of the flat portion increases on the basis of the radiuses Rs1 and Rs2 of the respective equalizing rings 22 and 24, and the reference spectrums measured at that time are shown in FIGS. 10A to 10E.
FIG. 10A shows a spectrum when Rp<Rs1, FIG. 10B shows a spectrum when Rp=Rs1, FIG 10C shows a spectrum when Rp=(Rs1+Rs2)/2, FIG. 10D shows a spectrum when Rp=Rs2, and FIG. 10E shows a spectrum when Rp>Rs2.
As can be seen from FIGS. 10A to 10E, when the radius Rp of the flat portion of the pole piece is large, the generation of the side band is correspondingly reduced, and thus a good spectrum is obtained. Actually, when measuring a noise level in the vicinity of a frequency of 2.4 GHz, the noise level is rapidly attenuated if the radius Rp of the flat portion is larger than the radius Rs1 of the outer circumference of the small-diameter equalizing ring 22, as shown in FIG. 11.
Therefore, in the conventional art, from this point of view, the radius Rp of the flat portion of the pole piece is generally set to be equal to or larger than the radius Rs2 of the inner circumference of the large-diameter equalizing ring 24, thereby preventing the leakage of unnecessary waves.
Further, as a countermeasure for noise, there has been proposed a method in which the length of the anode vane in the axial direction is set to be smaller than 70% of the minimum length between the pole pieces in the axial direction (between central flat portions), so that the distribution of the strength of a magnetic filed in the operation space is uniformed in the axial direction, thereby reducing a so-called line noise (for example, see Japanese Unexamined Patent Application Publication No. 6-223729).
As described above, in the conventional magnetron, the radius Rp of the flat portion of the pole piece is set to be equal to or larger than the radius Rs2 of the inner circumference of the large-diameter equalizing ring 24, thereby preventing the leakage of unnecessary waves. However, such a structure has another problem in that oscillation efficiency deteriorates on the other side.
Further, in the magnetron described in Patent Document 1, a reduction in line noise is achieved, but oscillation efficiency is not improved.
In order to prevent the leakage of unnecessary waves and to improve oscillation efficiency, the present inventors analyzed the relationship between the minimum length between the upper and lower pole pieces in the axial direction and the radius of each anode vane or each equalizing ring in detail, and obtained new knowledge.